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No, a girl can't get pregnant through sperms getting inside through the mouth. To get pregnant, sperms needs to meet ovum and that can happen through unprotected sexual intercourse only. Through mouth sperms will enter the oral cavity and may reach stomach and will die or excrete out, so they don't meet ovum anyway, in this tract.
Oral sex is considered as unnatural method of sex, and is highly discouraged. However, as far as your query is concerned, this doesn't cause pregnancy.
I hope it helps.
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